Electric contact combination



Patented Sept. 3, 1940 PATENT OFFICE ELECTRIC CONTACT COMBINATION FranzR. Hensel and Kenneth L. Emmett, In-

dianapolis, Ind., assig'nors to P. R. Mallory & 00., Inc., Indianapolis,Ind., a corporation of Delaware No Drawing, Application February 20,1940, Serial No. 319,888

4 Claims.

This invention relates to electrical contacts.

An object of the invention is to improve the combinations ofcompositions for electrical make and break contacts.

Another object of the invention is to provide a contact combinationwhich can be operated without objectionable contact metal transfer inautomobile electrical systems.

A still further object is to provide a contact metal combination whichwill retain a low contact resistance.

Other objects of the invention will be apparent from the followingdescription, taken in connection with the appended claims.

' The present invention comprises a combination of elements, methods ofmanufacture and products thereof, brought out and exemplified in thedisclosure hereinafter set forth, the scope of the -invention beingindicated in the appended claims,

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the method ofprocedure and combination of elements, without departing from the spiritof the invention.

Up to the present time, electrical contacts which are used, particularlyfor such applications as ignition systems, are made from pure tungstenand it is generally understood that the highest purity tungsten willgive the best contact performance, particularly as far as metal transferis concerned.

We have found that we can improve upon the contact performance of puretungsten in D. C. circuits, particularly automotive ignition circuits,by providing a differential contact combination, wherein the positivecontact consists of pure tungsten (at least 99.9% tungsten) and thenegative contact consists of an alloy of tungsten and molybdenum.

We have found that binary alloys of tungsten and molybdenum show anincreased hardness and therewith resistant to mechanical abrasion. Atthe same time they have shown considerable resistance to materialtransfer or build-up. We are aware of the factthat tungsten-molybdenumalloys, however, have been previously made for a number of purposes. Thealloys, have, however, never obtained any commercial importance in thefleld of electrical contacts, due to the fact that higher molybdenumcontents have a tendency to produce excessive contact resistance values.This was particularly noticeable if the molybdenum content exceeded 10%.

Tests have been run on tungsten contact combinations, where the positivecontact consisted of (Cl. 200-166) v pure tungsten, having a grain countof approximately 40,000 to 50,000 grains per square millimeter, and thenegative contact consisted of a tungstenmolybdenum alloy which in thepreferred case had a composition of 99.75% tungsten and .25% molybdenum.The grain size of the alloy varied from approximately 10,000 to 30,000grains per square millimeter.

Standard comparison tests on several automotive ignition systems wereconducted, wherein the distributor was operated at cam speeds of from550 to 750 R. P. M., with a primary applied voltage of 8.5 volts at aprimary peak current of 5.4 to 5.7 amperes, the contact or primary gapspacing being .020, with secondary gap spacing of .375", which secondarygap discharged in air, the duration of these tests being in the order of650 to 1000 continuous operating hours. I

Based on the above testing procedure, a standard tungsten versustungsten combination of the prior art gave an operating efficiency offrom '70 to determined by measuring the area under the curve produced byplotting the number of satisfactory units in operation versus time,while the combination of the present invention, based on the same methodof comparison, gave efficiency 100% on the above test would be producedby all units starting a test and operating satisfactorily, both from thestandpoint of metal transfer and contact resistance for the totalduration of the test.)

It has also been found that the combination of the present invention ismuch superior to a contact combination of tungsten-molybdenum alloyversus tungsten-molybdenum alloy. The operating efiiciency, with thealloy used for both contacts was found to be 25 to 30% lower than thecombination of the tungsten-molybdenum alloy versus pure tungsten.Moreover, if the polarity is reversed so that pure tungsten is used forthe negative contact and tungsten-molybdenum alloy for the positivecontact the efficiency obtained is 20 to 40% below that of thecombination contemplated by the invention.

Tests of the type described were carried out regularly over the periodof nearly a year and have confirmed that, on electric systems whereinmaterial transfer or build up is the main factor to be considered, theapplication of the combinations contemplated by the present inventionwill produce operating characteristics far more satisfactory than waspossible with prior art conditions.

In D. C. circuits, the positive contact or the values of from to 96%.(Note: A rating of anode generally tends to oxidize more quickly andmore severely than the cathode. This condition of over-oxidization isexperienced in ignition systems when operated at low temperatures. Theoxidization under these conditions is primarily due to increased burdensacross the contact points, caused by radical reduction in coilresistance values. Furthermore, there seems to exist certainelectrochemical reactions, causing a certain type of anodic oxidization.The contact combinations of the present invention were tested in systemsoperating in cold boxes at -10 F. and were found 100% satisfactory forperiods in excess of 500 continuous hours of operation, whereas contactsof the prior art such as tungsten versus tungsten had numerous failuresunder the same conditions.

The tests were extended to materials having considerably highermolybdenum contents and in some cases, the molybdenum was raised up to10%. By adjusting to electrical circuit conditions, it was possible toobtain the improved contact performance with a combination having as onecontact a tungsten-molybdenum alloy containing up to 10% molybdenum.

The contact combinations containing molybdenum in percentages up to 10%were found very satisfactory for direct current applications, where thepotential was in the order of 24 to 110 volts. In arriving at the bestpossible combinations, also the quality of the pure tungsten was variedand materials were used of different grain counts, varying from a graincount of 10 grains per square millimeter up to 100,000 grains per squaremillimeter. Also materials were tried on the pure tungsten side whichhad received special heat treatments, such as low temperature anneals ina hydrogen atmosphere. It was found that the various pure tungstenmaterials could be satisfactorily applied, providing that the purity ofthe tungsten was high enough. The tests indicated, however, that themost satisfactory performance of this new combination was obtained witha pure tungsten of a comparatively small grain size.

The tungsten-molybdenum alloy according to the invention may have apermissible range of composition of Molybdenum per cent .05 to 10Tungsten balance Most of the preferred compositions will have amolybdenum content of .05 to 2.5% and more preferably from .1 to .5%.

The tungsten-molybdenum alloys may be produced by well known andstandard methods such as by sintering the metal powders together, andalternately swaging and heat treating.

While the present invention as to its objects and advantages has beendescribed herein as carried out in specific embodiments thereof, it isnot desired to be limited thereby, but it is intended to cover theinvention broadly, within the spirit and scope of the appended claims.

What is claimed is:

1. An electric contact combination for D. C. operation in automotiveignition circuits, the contact on the positive side of said combinationbeing pure tungsten and the contact for the negative side being formedfrom an alloy of tungsten and molybdenum containing .05 to 10% ofmolybdenum.

2. An electric contact combination for D. C. operation in automotiveignition systems, the contact on the positive side of said combinationbeing pure tungsten and the contact for the negative side being formedfrom an alloy of tungsten and molybdenum, containing .1 to molybdenum.

3. An electric contact combination for D. C. operation in automotiveignition systems, the contact on the positive side of said combinationbeing pure tungsten and the contact for the negative side being formedfrom an alloy of tungsten and molybdenum, containing 25% molybdenum.

4. In an automobile electrical system, a direct current make and breakcontact combination comprising a contact of tungsten on the positiveside and a cooperating contact of tungsten containing from approximately.05 to 2.5% molybdenum on the negative side, said contact combinationbeing capable of operation in low temperatures without excessiveoxidation and metal transfer.

FRANZ R. HENSEL. KENNETH L. EMMERT.

